The fundamental purpose of this research is to determine the neuronal connections of the lower auditory system of the cat. This information is essential for the rational and systematic study of how lower auditory nuclei and their constituent cell types function in the hearing process. Previous work has defined the basic pattern of efferent projections from the several major subdivisions of the cochlear nucleus. In this prrposal the emphasis is on an investiation of both afferent and efferent connections of morphologically defined cell types in the cochlear nucleus by means of a combination of light microscopic techniques. More specifically, a mixture of tritiated amino acid and horseradish peroxidase (HRP) will be injected into specified small regions of the cochlear nucleus in order to determine both the efferent projections (by anterograde axonal transport of labelled protein) and the afferent connections (by retrograde axonal transport of HRP) of the cells in the injection site. By combining these two techniques on the same tissue section, reciprocal connections can be detected. The morphology of neurons which are the source of a specific projection or fiber bundle will be determined in a parallel series of experiments by making injections of origin in the cochlear nucleus. In addition, preparation of a detailed atlas of the superior olivary complex based on Nissl, Protargol, Golgi, and cholinesterase stained material will aid in specifying the detailed circuitry of the lower auditory system.